Method and apparatus for transmitting management information in wireless local area network system

ABSTRACT

A method for transmitting load information in a wireless local area network. The method according to one embodiment includes generating, by an access point (AP), basic service set (BSS) load information, the BSS load information including a multiple input multiple output (MIMO) channel underutilization field; and transmitting, by the AP, the BSS load information. The MIMO channel underutilization field indicates a spatial stream underutilization that is defined as a percentage of time that the AP has one or more underutilized spatial domain resources for a given busy time of a wireless medium. The spatial stream underutilization is calculated based on a maximum number of spatial streams supported by the AP and a number of one or more utilized spatial streams transmitted by the AP.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 13/264,340 filed on Oct. 13, 2011, which is the national phaseof PCT International Application No. PCT/KR2011/004818 filed on Jun. 30,2011, which claims the priority benefit under 35 U.S.C. §119(e) of U.S.Provisional Application Nos. 61/359,836 filed on Jun. 30, 2010,61/374,625 filed on Aug. 18, 2010, 61/417,894 filed on Nov. 30, 2010 and61/431,388 filed on Jan. 10, 2011. The entire contents of all of theabove applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method for transmitting management information in awireless local area network (WLAN) system and an apparatus forsupporting the method.

2. Discussion of the Related Art

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e. a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted. In the initial WLAN technology, a frequency of 2.4 GHz wasused according to the IEEE 802.11 to support a data rate of 1 to 2 Mbpsby using frequency hopping, spread spectrum, infrared communication,etc. Recently, the WLAN technology can support a data rate of up to 54Mbps by using orthogonal frequency division multiplex (OFDM). Inaddition, the IEEE 802.11 is developing or commercializing standards ofvarious technologies such as quality of service (QoS) improvement,access point protocol compatibility, security enhancement, radioresource measurement, wireless access in vehicular environments, fastroaming, mesh networks, inter-working with external networks, wirelessnetwork management, etc. The IEEE 802.11n is a technical standardrelatively recently introduced to overcome a limited data rate which hasbeen considered as a drawback in the WLAN. The IEEE 802.11n is devisedto increase network speed and reliability and to extend an operationaldistance of a wireless network.

The IEEE 802.11n supports a high throughput (HT), i.e., a dataprocessing rate of up to 540 Mbps or higher, and is based on a multipleinput and multiple output (MIMO) technique which uses multiple antennasin both a transmitter and a receiver to minimize a transmission errorand to optimize a data rate. In addition, this standard may use a codingscheme which transmits several duplicate copies to increase datareliability and also may use the OFDM to support a higher data rate.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing ratesupported by the IEEE 802.11n. However, an IEEE 802.11n medium accesscontrol (MAC)/physical layer (PHY) protocol is not effective to providea throughput of 1 Gbps or higher. This is because the IEEE 802.11nMAC/PHY protocol is designed for an operation of a single station (STA),that is, an STA having one network interface card (NIC), and thus when aframe throughput is increased while conforming to the conventional IEEE802.11n MAC/PHY protocol, a resultant additional overhead is alsoincreased. Consequently, there is a limitation in increasing athroughput of a wireless communication network while conforming to theconventional IEEE 802.11n MAC/PHY protocol, that is, a single STAarchitecture.

Therefore, to achieve a data processing rate of 1 Gbps or higher in thewireless communication system, a new system different from theconventional IEEE 802.11n MAC/PHY protocol (i.e., the single STAarchitecture) is required. A very high throughput (VHT) WLAN system is anext version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11WLAN systems which have recently been proposed to support a dataprocessing rate of 1 Gbps or higher in a MAC service access point (SAP).

The VHT WLAN system allows simultaneous channel access of a plurality ofVHT non-AP STAs for the effective use of a radio channel. For this, amulti-user multiple input multiple output (MU-MIMO)-based transmissionusing multiple antennas is supported. A VHT access point (AP) canconcurrently transmit spatial-multiplexed data to a plurality of VHTnon-AP STAs. When data is concurrently transmitted by distributing aplurality of spatial streams to the plurality of non-AP STA through aplurality of antennas, an overall throughput of the WLAN system can beincreased.

In the WLAN system, the non-AP STA performs scanning, authentication,and association procedures on an AP that provides a service. If aplurality of APs are found as a result of AP scanning performed by thenon-AP STA, the non-AP STA can select an AP to be associated. In thiscase, load balancing of an overall network is preferably taken intoaccount when the non-AP STA selects the AP.

A plurality of APs are installed as WLAN terminals are widespread andits utilization is increased. Accordingly, an overlapping basic serviceset (OBSS) environment increases in which a basic service area (BSA) ofa basic service set (BSS) using the same channel overlaps either partlyor wholly. In addition, in case of a WLAN supporting MU-MIMO, there maybe more considerations when selecting an AP to be associated with anon-AP STA. Therefore, it may be very important to provide the non-APSTA with information that can be used in a process of selecting the APto be associated with the non-AP STA in terms of managing overallefficiency of the WLAN.

In order to increase the overall efficiency of the WLAN, there is a needfor a method for generating control information that can be used by anon-AP STA when selecting an AP to be associated with the non-AP STA andfor transmitting the control information to the non-AP STA and also amethod for selecting the AP by the non-AP STA on the basis of thecontrol information.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for transmittinginformation of spatial stream under-utilization by an access point (AP)in a wireless local area network (WLAN) system.

The present invention also provides a method for determining an AP to beassociated with a non-AP station (STA) and an apparatus for performingthe method.

In an aspect of the present invention, a method of an access point (AP)selection in wireless local area network system includes receiving, froman AP in a candidate basic service set (BSS), a frame including a BSSload information element, the BSS load element including a multi user(MU) multiple input multiple output (MIMO) capable STA count field and aspatial stream utility field, wherein the MU-MIMO capable STA countfield indicates the total number of STAs with MU reception capabilitycurrently associated with the candidate BSS and the Spatial StreamUtility field indicates under utilized spatial streams for busy time ofwireless medium, determining a target BSS with which the stationassociates, based on the information indicated by the BSS loadinformation element.

The method may further include transmitting a probe request frame forscanning the candidate BSS and the frame may be a probe response frametransmitted in response to the probe request frame by the AP.

The Spatial Stream Utility field may be defined as the fraction of timethat the AP has one or more under utilized spatial stream for busy timeof the wireless medium.

The fraction of time may be linearly scaled with 255.

The frame may be a beacon frame broadcasted periodically.

In another aspect of the present invention, A method of transmittingmanagement information, performed by an access point, in wireless localarea network system includes transmitting, to a station, a frameincluding a BSS load information element, the BSS load element includinga multi user (MU) multiple input multiple output (MIMO) capable STAcount field and a spatial stream utility field, wherein the MU-MIMOcapable STA count field indicates the total number of STAs with MUreception capability currently associated with the candidate BSS and theSpatial Stream Utility field indicates under utilized spatial streamsfor busy time of wireless medium.

The method may further include receiving, from the station, a proberequest frame for scanning candidate BSSs, and the frame may be a proberesponse frame transmitted in response to the probe request frame.

The Spatial Stream Utility field may be defined as the fraction of timethat the AP has one or more underutilized spatial stream for busy timeof the wireless medium.

The fraction of time may be linearly scaled with 255.

The method may further include performing carrier sense mechanism fordetermining state of the wireless medium used to configure the spatialstream utility field.

The station may determine a target BSS with which the stationassociates, based on the information indicated by the BSS loadinformation element.

The frame may be a beacon frame broadcasted periodically.

In still another aspect of the present invention, an access point (AP)in wireless local area network system includes a processor configured toperform carrier sense mechanism for determining state of the wirelessmedium used to configure the spatial stream utility field, receive, fromthe station, a probe request frame for scanning candidate BSSs andtransmit, to a station, a probe response frame including a BSS loadinformation element in response to the probe request frame, the BSS loadelement including a multi user (MU) multiple input multiple output(MIMO) capable STA count field and a spatial stream utility field,wherein the MU-MIMO capable STA count field indicates the total numberof STAs with MU reception capability currently associated with thecandidate BSS and the Spatial Stream Utility field indicates underutilized spatial streams for busy time of wireless medium.

According to the present invention, available resource information of anaccess point (AP) can be reported to a non-AP station (STA), and thenon-AT STA can use the available resource information of the AP whenselecting the AP to be associated with the non-AP STA, thereby beingable to increase efficiency of a wireless local area network (WLAN).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an IEEE 802.11 physical layer architecture.

FIG. 2 shows an example of a BSS load information element formattransmitted by being included in a management frame.

FIG. 3 shows an example of a BSS load IE including channel correlationinformation according to an embodiment of the present invention.

FIG. 4 shows an example of procedure for providing the BSS load IE ofFIG. 3 by an AP to a non-AP STA.

FIG. 5 is an example of a BSS load IE format according to anotherembodiment of the present invention.

FIG. 6 shows a utilization of a channel used from the perspective of anAP.

FIG. 7 shows a method of expressing a MU-MIMO utilization according toan embodiment of the present invention.

FIG. 8 shows another example of a method of expressing a MU-MIMOutilization according to an embodiment of the present invention.

FIG. 9 shows another example of a method of expressing a MU-MIMOutilization according to an embodiment of the present invention.

FIG. 10 shows an example of a BSS load IE format including MU-MIMOutilization according to an embodiment of the present invention.

FIGS. 11 and 12 show an example of a BSS load IE format.

FIG. 13 shows another example of a BSS load IE format.

FIGS. 14 and 15 show an example of computing an average spatial streamcount in a metric manner.

FIG. 16 shows an example of transmission when interference occurs in aspecific band.

FIG. 17 shows a BS load IE format according to an embodiment of thepresent invention.

FIG. 18 shows an example of a BSS load IE according to an embodiment ofthe present invention when channel utility information is reported foreach bandwidth.

FIG. 19 shows an example of a BSS load IE format when reporting both aBW idle metric and a BW busy metric.

FIG. 20 is a block diagram showing a radio apparatus for implementing anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings.

A wireless local area network (WLAN) system according to an embodimentof the present invention includes at least one basic service set (BSS).The BSS is a set of stations (STAs) successfully synchronized tocommunicate with one another. The STA is any functional medium includinga medium access control (MAC) and wireless-medium physical layer (PHY)interface satisfying the institute of electrical and electronicsengineers (IEEE) 802.11 standard. The STA may be an AP or a non-AP STA.

An AP is any entity that has STA functionality and provides access to adistribution services (DS) via a wireless medium (WM) for associatedSTAs. The AP can also be referred to as other terms, such as acentralized controller, a base station (BS), a scheduler, etc.

The non-AP STA is an STA other than an AP, and can be referred to asother terms, such as a user equipment (UE), a mobile station (MS), amobile terminal (MT), a portable terminal, an interface card, etc.

The BSS can be classified into an independent BSS (IBSS) and aninfrastructure BSS. The infrastructure BSS includes at least one non-APSTA and AP.

FIG. 1 shows an IEEE 802.11 physical layer architecture.

The IEEE 802.11 physical (PHY) layer architecture includes a PHY layermanagement entity (PLME), a physical layer convergence procedure (PLCP)sub-layer 110, and a physical medium dependent (PMD) sub-layer 100. ThePLME provides a PHY layer management function in cooperation with a MAClayer management entity (MLME). The PLCP sub-layer 110 located between aMAC sub-layer 120 and the PMD sub-layer 100 delivers to the PMDsub-layer 100 a MAC protocol data unit (MPDU) received from the MACsub-layer 120 under the instruction of the MAC layer 120, or delivers tothe MAC sub-layer 120 a frame received from the PMD sub-layer 100. ThePMD sub-layer 100 is a lower layer of the PLCP sub-layer and serves toenable transmission and reception of a PHY layer entity between two STAsthrough a radio medium.

The PLCP sub-layer 110 attaches an additional field includinginformation required by a PHY transceiver to the MPDU in a process ofreceiving the MPDU from the MAC sub-layer 120 and delivering the MPDU tothe PMD sub-layer 100. The additional field attached in this case may bea PLCP preamble, a PLCP header, tail bits required on a data field, etc.The PLCP preamble serves to allow a receiver to prepare asynchronization function and antenna diversity before a PLCP servicedata unit (PSDU=MPDU) is transmitted. The PLCP header includes a fieldthat contains essential information for receiving and restoring a frameby a reception STA.

The PLCP sub-layer 110 generates a PLCP protocol data unit (PPDU) byattaching the aforementioned field to the MPDU and transmits thegenerated PPDU to the reception STA via the PMD sub-layer. The receptionSTA receives the PPDU, acquires information required for data recoveryfrom the PLCP preamble and the PLCP header, and recovers the data.

In order for the non-AP STA to participate in the WLAN, compatiblenetworks have to be identified. A scanning procedure is defined as aprocess in which the non-AP STA identifies a network that exists in aspecific region. In other words, the scanning procedure is a process offinding a candidate AP to be associated with the non-AP STA in anassociation or re-association procedure.

The scanning procedure has two types, i.e., passive scanning and activescanning. The passive scanning is a method of using a beacon frameperiodically transmitted by the AP. The non-AP STA may receive thebeacon frame, which is periodically transmitted by the AP that manages aBSS, to find an accessible BSS.

The active scanning is a method of finding an accessible BSS bytransmitting a probe request frame by the non-AP STA. In case of usingthe active scanning, when the non-AP STA transmits the probe requestframe, an AP that receives the probe request frame transmits to thenon-AP STA a probe response frame including information such as aservice set ID of the BSS managed by the AP, capability supported by theAP, etc. By the use of the received probe response frame, the non-AP STAcan know a variety of information regarding the candidate AP togetherwith the existence of the candidate AP.

The non-AP STA can know whether a joinable BSS exists by using thereceived beacon frame or the probe response frame in the scanningprocedure.

The scanning procedure is followed by an authentication procedure fornegotiating au authentication scheme and an encryption scheme betweenentities participating in wireless communication. For example, thenon-AP STA can perform the authentication procedure with an AP to beassociated among one or more APs found in the scanning procedure. Theauthentication procedure can use various schemes such as open-systemauthentication, shared-key authentication, pro-authentication,proprietary public-key authentication based on an algorithm developed bya vendor, etc. Examples of a further reinforced authentication schemeinclude IEEE 802.1x-based Extensible Authentication Protocol-TransportLayer Security (EAP-TLS), Extensible Authentication Protocol-TunneledTransport Layer Security (EAP-TTLS), Extensible AuthenticationProtocol-Flexible Authentication via Secure Tunneling (EAP-FAST), andProtected Extensible Authentication Protocol (PEAP).

After authentication is successfully complete in the authenticationprocedure, the non-AP STA can perform an association procedure with theAP. The association procedure implies establishing of an identifiableconnection (i.e., radio link) between the non-AP STA and the AP.

In the association procedure, the non-AP STA transmits an associationrequest frame to the AP which successfully completes the authenticationprocedure, and in response thereto, the AP transmits an associationresponse frame having a status code of ‘successful’ to the non-AP STA.The association response frame includes an identifier (e.g., anassociation ID (AID)) capable of identifying an association with aspecific non-AP STA.

In a case where a connection status between the non-AP STA and the APdeteriorates due to a variable channel state even after the associationprocedure is successfully complete, the non-AP STA can perform again theassociation procedure with another AP having a good channel state, whichis referred to as a re-association procedure. The re-associationprocedure is very similar to the aforementioned association procedure.More specifically, in the re-association procedure, the non-AP STAtransmits a re-association request frame to a different AP (e.g., an APwhich successfully completes the authentication procedure amongcandidate APs found in the aforementioned scanning procedure) other thanan AP currently associated with the non-AP STA, and the different APtransmits a re-association response frame to the non-AP STA. However,the re-association request frame includes information on a previouslyassociated AP. By using this information, the re-associated AP candeliver data buffered in the previous AP to the non-AP STA.

Hereinafter, a method of determining a specific BSS to be joined will bedescribed in greater detail when a non-AP STA finds a plurality ofjoinable BSSs as a result of the aforementioned scanning procedure. Thedetermining of the specific BSS to which the non-AP STA will join canalso be expressed as determining of a specific AP on whichauthentication and association procedures will be performed.

When the non-AP STA selects one of a plurality of candidate BSSs towhich the non-AP STA can join and thus determines a BSS to be joined, itmay be preferable to determine the BSS by considering a load of eachcandidate BSS. Non-AP STA population and traffic levels of eachcandidate BSS can be used to determine the BSS to be joined so as toprevent a load to be concentrated in a specific BSS, thereby being ableto increase overall efficiency of a WLAN system. For this, eachcandidate BSS's load information that can be used by the non-AP STAneeds to be reported to the non-AP STA.

The AP can transmit a management frame including a load informationelement of the BSS in order to report state information of the AP to anSTA. The management frame including the load information element may beunicast to a non-AP STA performing scanning in the scanning procedure ormay be broadcast to all STAs within a basic service area (BSA) of theBSS. Alternatively, the management frame including the BSS load IE maybe broadcast periodically to a non-AP STA within the BSA.

FIG. 2 shows an example of a BSS load information element formattransmitted by being included in a management frame.

The BSS load information element contains information on the current STApopulation and traffic levels in the BSS. This information element maybe used by the non-AP STA for AP selection algorithm when roaming.

The BSS load information element of FIG. 2 includes an Element ID fieldincluding identification information of an information element (IE), aLength field including length information of the IE, an STA Count field,a Channel Utilization field, and an Available Admission Capacity field.

The STA Count field is interpreted as an unsigned integer that indicatesthe total number of STAs currently associated with this BSS.

The Channel Utilization field is defined as the percentage of time,linearly scaled with 255 representing 100%, that the AP sensed themedium was busy, as indicated by either the physical or virtual carriersense (CS) mechanism. When more than one channel is in use for the BSS,the Channel Utilization field value is calculated only for the primarychannel. This percentage is computed using the equation 1.

ChannelUtilization=Integer((channel_busy_time/(dot11ChannelUtilizationBeaconIntervals×dot11BeaconPeriod×1024))×255),  [Equation1]

where channel_busy_time is defined to be the number of microsecondsduring which the CS mechanism has indicated a channel busy indication,dot11ChannelUtilizationBeaconIntervals represents the number ofconsecutive beacon intervals during which the channel busy time ismeasured.

The Available Admission Capacity field may be 2 octets long and containsan unsigned integer that specifies the remaining amount of medium timeavailable via explicit admission control, in units of 32 μs/s. The fieldis helpful for roaming non-AP STAs to select an AP that is likely toaccept future admission control requests, but it does not represent aguarantee that the HC will admit these requests.

In the BSS load information element format of FIG. 2, AP's loadinformation (e.g., a spatial re-use factor of STAs, a channelcorrelation among the STAs, etc.) related to MIMO transmission is notincluded in a WLAN system supporting MIMO transmission. A non-AP STAwhich intends to select an AP in the WLAN system supporting MIMOtransmission is preferably capable of selecting the AP by alsoconsidering information of a spatial re-use factor of candidate APs, achannel correlation among non-AP STAs, etc. Hereinafter, transmissioninformation and a method of transmitting a variety of BSS loadinformation that can be transmitted by an AP to a non-AP STA in a WLANsystem supporting MU-MIMO will be described in detail by taking adetailed example.

In the following description of the present invention, the BSS load IEis an IE including control information transmitted by an AP to a non-APSTA in a BSA, and its name is for exemplary purposes only. Hereinafter,information transmitted by being included in the BSS load IE may betransmitted by being included in one IE or may be transmitted as aseparate IE.

The AP may transmit the BSS load IE by using a control frame and/or amanagement frame. The control frame and/or the management frame may beunicast to the non-AP STA or may be broadcast to all non-AP STAs withina BSA. The AP may transmit the BSS load IE at the request of informationprovision, or may transmit an unsolicited BSS load IE irrespective ofthe request of information provision. The management frame may be thebeacon frame or the probe response frame. The non-AP STA may select anoptimal AP on the basis of information obtained by using the BSS loadIE.

FIG. 3 shows an example of a BSS load IE including channel correlationinformation according to an embodiment of the present invention.

An AP transmits the BSS load IE to a non-AP STA. Herein, the BSS load IEincludes information such as Channel Correlation Threshold, Number ofCorrelated STAs, Number of Uncorrelated STAs, Channel Utilization ofCorrelated STA, and Channel Utilization of Uncorrelated STA.

FIG. 4 shows an example of procedure for providing the BSS load IE ofFIG. 3 by an AP to a non-AP STA.

In the example of FIG. 4, the non-AP STA transmits to an AP a requestframe for requesting channel correlation related information of FIG. 3to request a channel correlation IE in an active scanning procedure.Thereafter, in response to the request frame, the channel correlationrelated information is obtained by receiving a response frame includingthe BSS load IE (of the FIG. 3) containing the channel correlationrelated information. The non-AP STA can utilize the obtained informationin AP selection. As described above, the response frame may betransmitted without the request frame.

Hereinafter, the active scanning procedure will be described for twocases, i.e., a sounding PPDU based active scanning procedure and a nulldata packet (NDP) based active scanning procedure. The sounding PPDUbased active scanning procedure will be first described. When a non-APSTA transmits a probe request frame, an AP receiving the probe requestframe transmits a probe response frame in response to the probe requestframe. In this case, the probe response frame may include a trainingrequest (TRQ) message for requesting the non-AP STA to transmit thesounding PPDU. Upon confirming the TRQ message included in the proberesponse frame, the non-AP STA transmits to the AP a request frame forrequesting channel correlation related information. In this case, therequest frame may act as the sounding PPDU. That is, the AP can performchannel estimation by using the request frame. The AP transmits to thenon-AP STA a response frame including a BSS load IE in response to therequest frame for requesting the channel correlation relatedinformation. The non-AP STA receives the response frame and can useinformation obtained by using the BSS load IE to determine an AP to beauthenticated and associated with the non-AP STA.

In the NDP based active scanning procedure, the BSS load IE can beprovided as follows. The non-AP STA and the AP exchange the proberequest frame and the probe response frame. The non-AP STA transmits tothe AP a request frame for requesting the channel correlation relatedinformation. The request frame includes an NDP announcement message forreporting that the NDP will be transmitted subsequently. The non-AP STAtransmits the NDP to the AP subsequently to the request frame. The APperforms channel estimation by using the NDP, and transmits to thenon-AP STA the response frame including the BSS load IE containing thechannel correlation related information. A channel estimation result maybe considered in information transmitted by being included in the BSSload IE.

For another example, the AP can broadcast the BSS load IE irrespectiveof the request of the non-AP STA. The non-AP STA can select the AP byconsidering the broadcast BSS load IE. The BSS load IE which isbroadcast by the AP may include information of antenna utilization andbandwidth utilization together with channel utilization.

The antenna utilization may be a value indicating a utilization of ausage amount of a MU-MIMO spatial stream. The bandwidth utilization maybe a value indicating a utilization of a channel bandwidth in use.

FIG. 5 is an example of a BSS load IE format according to anotherembodiment of the present invention. The BSS load IE of FIG. 5 allows anon-AP STA for receiving the BSS load IE to be able to use a channelcorrelation between non-AP STAs supporting MU-MIMO and performance for acase where the non-AP STAs supporting MU-MIMO are co-scheduled.

The AP transmits the BSS load IE including a STA Count field, a ChannelUtilization field, and an Available Admission Capacity field. The non-APSTA may consider the BSS load IE in AP selection to achieve overallnetwork's load balancing.

In one embodiment, the AP may transmit a beacon frame by including theBSS load IE. In this case, the BSS load IE may include the STA Countfield, the Channel Utilization field, and the Available AdmissionCapacity field.

A non-AP STA supporting only SU-MIMO and a non-AP STA supporting MU-MIMOmay coexist within a BSS. From the perspective of the AP, a case oftransmitting and receiving data by utilizing a channel by the non-APSTAs supporting only SU-MIMO and a case of transmitting and receivingdata by utilizing a channel by the STAs supporting MU-MIMO have to beboth supported.

FIG. 6 shows a utilization of a channel used from the perspective of anAP.

A time window in which the AP measures a load status may include alegacy or SU-MIMO format channel busy period and idle period measured ina PHY layer, and may include a time duration in which an AP supportingMU-MIMO is serving data by using MU-MIMO.

In the time duration in which the AP transmits data by using MU-MIMO,the number of spatial streams (SSs) transmitted by the AP may not be themaximum transmissible number according to a situation. For example, itis assumed that there is only one STA supporting MU-MIMO, and the STAsupports only one SS. In this case, the AP cannot use more than one SSin the time duration of MU-MIMO transmission, and thus a system operateswith a system capacity lower than that can be serviced by the AP inpractice. As a result, usage efficiency of radio resources decreases.There is a need for a method for effectively using MU-MIMO by allowingthe AP supporting MU-MIMO to be able to fully utilize availablecapacity.

According to an embodiment of the present invention, the usageefficiency of radio resources can be increased by reporting a MU-MIMOutilization or a MU-MIMO under utilization to the non-AP STA. In theMU-MIMO (under) utilization according to the embodiment of the presentinvention, similar to the BSS load IE, a (under) utilization metricwhich considers a load in the reporting is calculated by utilizingspatial stream count information which is used (or not used) by the AP.That is, when calculating the MU-MIMO (under) utilization, reporting canbe performed by utilizing spatial stream information for MU-MIMO, acarrier sense (SS) busy time, time duration information for calculatingreporting, etc.

Hereinafter, calculation of the MU-MIMO (under) utilization will bedescribed in greater detail by taking an example.

FIG. 7 shows a method of expressing a MU-MIMO utilization according toan embodiment of the present invention.

In the method of expressing the MU-MIMO utilization according to theembodiment of the present invention, instead of simply expressingwhether a channel is busy or idle, a level of busyness is classifiedinto several levels. For example, when there are remaining availablespatial streams, it can be expressed by a low busy level, and when thereis no available remaining spatial stream, it can be expressed by amaximum busy level. When a channel is idle, it can be expressed by aminimum busy level, a carrier sense (CS) busy time in which an AP doesnot perform MU-MIMO transmission can be expressed by a maximum busylevel, and a utilization is expressed by an average busy level betweenthe two levels. The busy level can be computed by the equation 2.

MUMIMO_Channel_Utilization=Integer((channel_busy_level_time/(maximum_busy_level×dot11ChannelUtilizationBeaconIntervals×dot11BeaconPeriod×1024))×255)  [Equation2]

where

channel_busy_level_time is defined to be the number of microsecondsmultiplied by channel_busy_level during which the CS mechanism, asdefined in section 9.2.1 of IEEE 802.11-2007 specification, hasindicated a channel busy indication.

channel_busy_level is defined to be 0 for idle CS times,maximum_busy_level for CS instances where the AP did not transmit inMU-MIMO data transmission. channel_busy_level is between 0 andmaximum_busy_level and linearly scaled to number of utilized spatialstreams for CS instances where AP has transmitted in MU-MIMO datatransmission.

(e.g. 1 channel_busy_level is equal to utilized spatial stream andmaximum_busy_level is equal to maximum supportable spatial streams forSTAs in MU-MIMO transmission mode)

FIG. 8 shows another example of a method of expressing a MU-MIMOutilization according to an embodiment of the present invention.

In another exemplary method of setting a MU-MIMO under utilizationvalue, an under utilization is computed with several idle levels when anAP is busy due to MIMO data transmission, instead of expressing theunder utilization with a simply idle level. In other words, MU-MIMOunder utilization value may be defined as the fraction of time that APhas under utilized spatial domain resources (spatial stream) for busytime of the wireless medium. When more than one channel is in use forthe BSS, the spatial stream under utilization value may be calculatedonly for the primary channel. For example, when there are remainingavailable spatial streams, it is expressed by a high idle level. Whenthere is no available spatial stream, it is expressed by a minimum idlelevel or a busy level. An under utilization is expressed by an averageidle level between the two levels. The idle level can be computed by theequation 3.

MUMIMO_Channel_Under_Utilization=Integer((channel_idle_level_time/channel_MUMIMO_busy_time)×255)  [Equation3]

where

channel_MUMIMO_busy_time is defined to be the number of microsecondsduring the CS mechanism for MU-MIMO transmission at the AP.

channel_idle_level_time is defined to be the number of microsecondsmultiplied by channel_idle_level during which the CS mechanism, asdefined in section 9.2.1 of IEEE 802.11-2007 specification, hasindicated a channel busy indication.

(e.g. 1 channel_idle_level is equal to maximum supported spatialstream−utilized spatial stream and maximum_idle_level is equal tomaximum supportable spatial streams for a STA in MU-MIMO transmissionmode)

(e.g. 2 channel_idle_level is equal to min{maximum supported spatialstream for a single MU-MIMO STA, maximum supported spatialstream−utilized spatial stream} and maximum_idle_level is equal tomaximum supportable spatial streams for a single STA in MU-MIMOtransmission mode)

FIG. 9 shows another example of a method of expressing a MU-MIMOutilization according to an embodiment of the present invention.

In another exemplary method of expressing a MU-MIMO under utilization,the MIMO under utilization computes an under utilization with severalidle levels when a CS is idle and when an AP is busy due to MIMO datatransmission, instead of expressing the under utilization with a simplyidle level. For example, when there are remaining available spatialstreams, it is expressed by a high idle level. When there is noavailable spatial stream, it is expressed by a minimum idle level or abusy level. An under utilization is expressed by an average idle levelbetween the two levels. The idle level can be computed by the equation4.

MUMIMO_Channel_Under_Utilization=Integer((channel_idle_level_time/(channel_idle_time+channel_MUMIMO_busy_time)×255)  [Equation4]

where

channel_MUMIMO_busy_time is defined to be the number of microsecondsduring the CS mechanism for MU-MIMO transmission at the AP.

channel_idle_time is defined to be the number of microseconds during CSmechanism is not busy (i.e. idle).

channel_idle_level_time is defined to be the number of microsecondsmultiplied by channel_idle_level during which the CS mechanism, asdefined in section 9.2.1 of IEEE 802.11-2007 specification, hasindicated a channel busy indication.

(e.g. 1 channel_idle_level is equal to maximum supported spatialstream−utilized spatial stream and maximum_idle_level is equal tomaximum supportable spatial streams for a STA in MU-MIMO transmissionmode, utilized spatial stream is zero for idle time instances)

(e.g. 2 channel_idle_level is equal to min(maximum supported spatialstream for a single MU-MIMO STA, maximum supported spatialstream−utilized spatial stream) and maximum_idle_level is equal tomaximum supportable spatial streams for a single STA in MU-MIMOtransmission mode, utilized spatial stream is zero for idle timeinstances)

Hereinafter, a method of transmitting the aforementioned MU-MIMOutilization information to a non-AP STA will be described.

FIG. 10 shows an example of a BSS load IE format including MU-MIMOutilization according to an embodiment of the present invention.

Non-AP STAs not supporting MU-MIMO transmission and non-AP STAssupporting MU-MIMO transmission can coexist within a BSS. In order forboth the non-AP STAs not supporting MU-MIMO transmission and the non-APSTAs supporting MU-MIMO transmission to be able to obtain information ona load status of the BSS, an AP can transmit a BSS load element as shownin the example of FIG. 2, and can additionally transmit a supported BSSload element including the information on the load status of the BSS ina MU-MIMO transmission situation.

FIG. 11 and FIG. 12 show an example of a BSS load IE format.

Channel utilization information in MU-MIMO transmission is included in aMU-MIMO Channel Utilization field of FIG. 11 and a MU-MIMO Channel UnderUtilization field of FIG. 12. The channel utilization information can beexpressed by the number of spatial streams in use or the number ofremaining available spatial steams. This will be described below ingreater detail.

FIG. 13 shows another example of a BSS load IE format.

In addition, in a BSS load element according to an embodiment of thepresent invention, MU-MIMO channel under utilization information can betransmitted together along with MU-MIMO channel utilization information.

Meanwhile, any one channel can be used by a plurality of APs. Forexample, an overlapping BSS (OBSS) environment in which a BSA ofdifferent BSSs using the same channel overlaps partly or wholly can beassumed. In this environment, when any AP computes a load of a channel,it may be computed such that the channel is busy in a period in whichanother AP uses the channel. The AP can report information on a MU-MIMOload status (i.e., an un-used spatial stream count or a used spatialstream count) to non-AP STAs within a BSA of the AP only in a busy-timeduration in which the AP performs MU-MIMO transmission.

Even if the AP performs SU-MIMO transmission, information on the un-usedspatial stream count or the used spatial stream count can be reported tothe non-AP STAs within the BSA of the AP in a time duration in whichSU-MIMO transmission is achieved. When the AP transmits data to an STAnot supporting MU-MIMO, the time duration can be determined to abusy-time.

A BSS load IE to be transmitted by the AP in the BSS supporting MU-MIMOmay further include Current Throughput, Utilized Spatial Streams,Average Transmit bandwidth (BW), and Number of MU-MIMO capable STAs.

Current Throughput is a type of load status information, and can bedefined as follows.

“Load=num_transmit_bytes/max_num_transmit_bytes”

Both num_transmit_bytes, and max_num_transmit_bytes are number of bytesin a observed time window similar to what is defined in the BSS loadelement channel utility definition.

‘transmit_bytes’ may count bytes regardless of whether or not particularpacket has not been ACKed

The load is an indicator of how the medium is busy, so even if thetransmitted packet has not succeeded it still took a chunk of the mediumaccess time

In the definition of Current Throughout, packets are still counted evenif ACK is not received. This is to indicate a throughput used when achannel medium is consumed. However, since accurate AP's throughputinformation can be requested in some cases, the throughput can beexpressed in practice in throughput computation by counting only packetsfor which ACK is received and which are successfully received uponreceiving ACK.

In Utilized Spatial Streams, average transmission spatial streaminformation can be expressed by computing an average spatial streamcount in a metric manner within a time in which an AP sends a data PPDUto a MU-MIMO capable STA. FIG. 14 shows an example thereof.

“Load=average_num_transmit_ss1/max_num_transmit_ss”

In order to give insight on MU-MIMO capability, the‘average_num_transmit_ss1’ may be only computed using channel mediumbusy time in which the AP was serving data to MU-MIMO capable STAs

In another example of Utilized Spatial Streams, average transmissionspatial stream information can be expressed by computing an averagespatial stream count in a metric manner within a time in which an APsends a data PPD to a MU-MIMO capable STA, and can be expressed byaveraging a maximum spatial stream count defined in a medium busy periodin which the AP does not send the data PPDU to the MU-MIMO capable STA.FIG. 15 shows an example thereof.

“Load=average_num_transmit_ss2/max_num_transmit_ss”

In order to give insight on MU-MIMO capability, the‘average_num_transmit_ss2’ is computed across all channel medium busytime and may is equal to ‘max_num_transmit_ss’ when the channel mediumis busy because of other STA using the medium or AP serving data tonon-MU-MIMO capable STAs

For STAs, it will be beneficial to get ratio in which STA could havebeen in SU or MU-MIMO transmission mode and information on expectedunder-utilized spatial dimension at AP side

Utility measurement method in FIG. 14 lacks information on what wouldhave been the ratio between under-utilized busy medium status and fullyutilized busy medium status. SS utility measurement in FIG. 14 close to0 does not necessary mean MU-MIMO capable STAs were serviced frequentlynor does it mean any future STAs will be serviced frequently in MU-MIMO

Current Throughput may not give spatial domain utilization informationand may depict the wrong picture. Throughput loss can occur due toparticular STAs channel conditions (link adaptation issues). Lowthroughput has not relationship between MU-MIMO capable STAs and channelutility.

The average transmit bandwidth (BW) is information regarding a bandwidthof a channel used by the AP. Average information of a transmissionbandwidth utilized in the BSS may be delivered to the STA, whichfacilitates a process of selecting a BSS by the STA. This is because,even if channel utilization information of the same BSS load element ispresent, a transmission bandwidth in which a channel utility is used maydiffer on average in each BSS. In particular, when interference occursin a specific band, a full bandwidth cannot always be used, andtransmission and reception will be performed by partly adapting thebandwidth. Each BSS may have a different available bandwidth. FIG. 16shows an example thereof. If the primary subchannel is not affected bythe interference, medium access and load of the primary subchannel isunaffected. Average BW utilization may not be the same even with samelegacy BSS load utility

Transmission BW of an AP can be restricted by not only other BSS indifferent sub-channels but also 3rd party wireless signals in 5 GHzband. STAs wanting more throughput may want information an typical BWusage for a given BSS.

Number of MU-MIMO capable STAs denotes the number of non-AP STAssupporting MU-MIMO. MU-MIMO capable STAs might want to associate itselfwith BSS with more MU-MIMO capable advanced STAs in order to fullyexploit potential MU-MIMO benefits.

FIG. 17 shows a BS load IE format according to an embodiment of thepresent invention. Herein, a spatial stream utility metric is a MU-MIMOchannel utility metric. In this case, a medium busy time utilized for aMU-MIMO capable STA by an AP can be computed as spatial streaminformation actually used. Further, the remaining busy time can beassumed as maximum spatial stream information in computation.

The BSS load IE format may include a Spatial Stream Utility Metricfield, a Bandwidth Utility Metric field, and a MU-MIMO Capable STA Countfield. A length of each field of FIG. 17 is for exemplary purposes only,and thus can be increased or decreased if necessary when implemented. Afield transmission order and information included in the BSS load IEformat of FIG. 17 is information that can be included in the BSS load IEin the present invention, and can include the exemplified informationwholly or partly.

Hereinafter, each field will be described in greater detail.

Spatial Stream Utility Metric may be represented as below.

Spatial Stream Utility Metric=((spatial stream busy leveltime/(max-spatial-stream-busy-level*channel busy time))*255)

‘spatial stream busy level time’ is defined to be total sum of‘number-of spatial-streams’ multiplied by number of microseconds duringwhich CS mechanism has indicated channel busy indication

‘number-of-spatial-streams’ may be equal to transmitted number ofspatial streams during which the AP has occupied the medium to transmitPPDU to MU-MIMO capable STA(s), otherwise (i.e. other channel busyinstances) may be equal to ‘max-spatial-stream-busy-level’

Bandwidth Utility Metric may be represented as below.

Bandwidth Utility Metric=((transmit bandwidth busytime/(max-transmit-bandwidth*BSS medium busy time))*255)

‘transmit bandwidth busy time’ is defined to be total sum of‘transmit-bandwidth’ multiplied by number of microseconds during whichdevice in the BSS (i.e. AP or non-AP STA which is associated with theAP) has been detected to occupy the medium

‘transmit-bandwidth’ may be equal to transmitted PPDU bandwidth, Where‘BSS medium busy time’ is defined as number of microseconds during whichdevice in the BSS (i.e. AP or non-AP STA which is associated with theAP) has been detected to occupy the medium

Number of MU-MIMO capable STAs may be interpreted as an unsigned integerthat indicates the total number of STAs with MU-MIMO transmissionreception capability currently associated with this BSS.

Several measurement problems may arise in a process of reportinginformation on a bandwidth usable or available in any BSS. For example,measurement is impossible while an AP transmits a specific signal. Inaddition, according to implementation, there is a case where measurementcannot be performed simultaneously while decoding a PPDU which is beingreceived. In order to solve the several problems, a bandwidth-relatedmetric will be defined and a method of utilizing the metric will bedescribed hereinafter according to an embodiment of the presentinvention.

Bandwidth Utility Metric according to the embodiment of the presentinvention can be reported in a state where a channel medium is idle(herein, idle means that a primary channel is idle in the specific BSS)by measuring whether a channel is idle for each available bandwidth,which can be used by a specific BSS in an idle state. In this case, theavailable bandwidth that can be used by the BSS may be 20 MHz, 40 MHz,80 MHz, 160 MHz, or 80+80 MHz. For example, through channel carriersensing, each idle time is computed for various bandwidth such as20/40/80/160 by including a primary subchannel, and a computation resultis reported for each bandwidth. Reporting for each bandwidth may be away of indicating a maximum bandwidth in an idle state.

In computation, a bandwidth utility value for a wider bandwidth mayfully include an idle time computed in a smaller bandwidth utility. Forexample, if it is determined that a bandwidth of 40 MHz is idle, it isnatural that a bandwidth of 20 MHz is idle. Thus, a 40 MHz idle time canbe included when computing a 20 MHz idle time. Alternatively, the 20 MHzidle time may not include the 40 MHz idle time in order to deliver ametric indicating that a channel medium other than 20 MHz was busy forother available bandwidths. Bandwidth Utility Metric can be reported bycomputing an average value of a metric which is idle for each bandwidth.

According to another embodiment, Bandwidth Utility Metric can becomputed by using a bandwidth of a PLCP protocol data unit (PPDU) to betransmitted immediately before transmission of a specific PPDU by an APin a BSS.

In a specific band other than a band in which transmission is performed,the AP cannot know whether another BSS uses the specific band while aPPDU is being transmitted. Therefore, it is difficult to compute anaccurate bandwidth utility while the PPDU is transmitted.

Under the assumption that a status (i.e., whether it is an idle or not)for each bandwidth of a channel medium is maintained while the PPDU istransmitted, Bandwidth Utility Metric can be computed by using abandwidth used in transmission of the PPDU.

When a specific PPDU is received by an AP in a BSS, a method ofmeasuring a bandwidth spanned by the received PPDU and for computingwhether a medium is busy for each bandwidth can be used. An averagevalue of a metric which is busy for each bandwidth can be reported.

According to an embodiment, Bandwidth Utility Metrics obtained by theaforementioned two methods can be both reported or a sum of BandwidthUtility Metrics can be reported. By transmitting both Bandwidth UtilityMetrics or by reporting the sum of Bandwidth Utility Metrics, STAs to beassociated can fully recognize whole information regarding an idle andbusy time according to corresponding information, thereby being able toselect a suitable AP.

FIG. 18 shows an example of a BSS load IE according to an embodiment ofthe present invention when channel utility information is reported foreach bandwidth.

The BSS load IE may include a MU-MIMO capable STA Count field, a SpatialStream Utility field and a plurality of BW utility fields. The MU-MIMOcapable STA Count field indicates the total number of STAs with MUreception capability currently associated with the BSS. The STAs with MUreception capability support MU-MIMO transmission and/or reception. TheSpatial Stream Utility field indicates the state of resourceutilization. The state of resource utilization may be computed by theeach method described with FIG. 6˜17.

BW utility is reported for each bandwidth. Time information in a statewhere corresponding information is idle is reported for each bandwidth.A field containing information on a bandwidth not supported in acorresponding BSS can be omitted in a generation procedure or atransmission procedure of the BSS load IE.

Bandwidth Idle Utility Metric may be represented as below.

Bandwidth Idle Utility Metric=((idle_time_per_bandwidth/(BSS medium idletime))*255)

‘idle_time_per_bandwidth’ is defined to be number of microseconds duringwhich either 20/40/80/160 MHz bandwidth has been detected to be idle,

Where ‘BSS medium idle time’ is defined as number of microseconds duringwhich AP detected the medium of the primary channel to be idle.

When BW utility is information about busy time of each of the channelbandwidth, Bandwidth Busy Utility Metric may be represented as below.

Bandwidth Busy Utility Metric=((busy_time_per_bandwidth/(BSS medium busytime))*255)

‘busy_time_per_bandwidth’ is defined to be number of microseconds duringwhich device in the BSS (i.e. AP or non-AP STA which is associated withthe AP) has been detected to occupy the medium for either 20/40/80/160MHz bandwidth

Where ‘BSS medium busy time’ is defined as number of microseconds duringwhich device in the BSS (i.e. AP or non-AP STA which is associated withthe AP) has been detected to occupy the medium

FIG. 19 shows an example of a BSS load IE format when reporting both aBW idle metric and a BW busy metric.

FIG. 20 is a block diagram showing a radio apparatus for implementing anembodiment of the present invention. A radio apparatus 2000 may be an APor a non-AP STA.

The radio apparatus 2000 includes a processor 2010, a memory 2020, atransceiver 2030, and multiple antennas 2050. The transceiver 2030 isconfigured to transmit and/or receive a management frame of the presentinvention. The processor 2010 is functionally coupled to the transceiver2030 and is configured to generate and process the management frame. Theprocessor 2010 and the transceiver 2030 implement a PHY layer and a MAClayer of IEEE 802.11. The processor 2010 and/or the transceiver 2030 mayinclude an application-specific integrated circuit (ASIC), a separatechipset, a logic circuit, and/or a data processing unit. The memory 2020may include a read-only memory (ROM), a random access memory (RAM), aflash memory, a memory card, a storage medium, and/or other equivalentstorage devices. When the embodiment of the present invention isimplemented in software, the aforementioned methods can be implementedwith a module (i.e., process, function, etc.) for performing theaforementioned functions. The module may be stored in the memory 2020and may be performed by the processor 2010. The memory 2020 may belocated inside or outside the processor 2010, and may be coupled to theprocessor 2010 by using various well-known means.

The aforementioned embodiments include various exemplary aspects.Although all possible combinations for representing the various aspectscannot be described, it will be understood by those skilled in the artthat other combinations are also possible. Therefore, all replacements,modifications and changes should fall within the spirit and scope of theclaims of the present invention.

What is claimed is:
 1. A method for transmitting load information in awireless local area network, the method comprising: generating, by anaccess point (AP), basic service set (BSS) load information, the BSSload information including a multiple input multiple output (MIMO)channel underutilization field; and transmitting, by the AP, the BSSload information, wherein the MIMO channel underutilization fieldindicates a spatial stream underutilization that is defined as apercentage of time that the AP has one or more underutilized spatialdomain resources for a given busy time of a wireless medium, and whereinthe spatial stream underutilization is calculated based on a maximumnumber of spatial streams supported by the AP and a number of one ormore utilized spatial streams transmitted by the AP.
 2. The method ofclaim 1, wherein the BSS load information further includes a multiuser-multiple input multiple output (MU-MIMO) capable STA count field,and the MU-MIMO capable STA count field indicates a total number ofMU-capable STAs that are currently associated with a BSS of the AP andhave capabilities of MU-MIMO reception.
 3. The method of claim 1,wherein the BSS load information is included in a probe response frameor a beacon frame.
 4. The method of claim 1, wherein the percentage oftime for the spatial stream underutilization is linearly scaled with255.
 5. A device for a wireless local area network system, the devicecomprising: a transceiver configured to transmit radio signals; and aprocessor configured to: generate basic service set (BSS) loadinformation, the BSS load information including a multiple inputmultiple output (MIMO) channel underutilization field, and instruct thetransceiver to transmit the BSS load information, wherein the MIMOchannel underutilization field indicates a spatial streamunderutilization that is defined as a percentage of time that the devicehas one or more underutilized spatial domain resources for a given busytime of a wireless medium, and wherein the spatial streamunderutilization is calculated based on a maximum number of spatialstreams supported by the device and a number of one or more utilizedspatial streams transmitted by the device.
 6. The device of claim 5,wherein the BSS load information further includes a multi user-multipleinput multiple output (MU-MIMO) capable STA count field, and the MU-MIMOcapable STA count field indicates a total number of MU-capable STAs thatare currently associated with a BSS of the AP and that have capabilitiesof MU-MIMO reception.
 7. The device of claim 5, wherein the BSS loadinformation is included in a probe response frame or a beacon frame. 8.The device of claim 5, wherein the percentage of time for the spatialstream underutilization is linearly scaled with 255.